RESUMEN
Nanomechanical resonators realized from tensile-strained materials reach ultralow mechanical dissipation in the kHz to MHz frequency range. Tensile-strained crystalline materials that are compatible with epitaxial growth of heterostructures would thereby at the same time allow realizing monolithic free-space optomechanical devices, which benefit from stability, ultrasmall mode volumes, and scalability. In our work, we demonstrate nanomechanical string and trampoline resonators made from tensile-strained InGaP, which is a crystalline material that is epitaxially grown on an AlGaAs heterostructure. We characterize the mechanical properties of suspended InGaP nanostrings, such as anisotropic stress, yield strength, and intrinsic quality factor. We find that the latter degrades over time. We reach mechanical quality factors surpassing 107 at room temperature with a Q·f product as high as 7 × 1011Hz with trampoline-shaped resonators. The trampoline is patterned with a photonic crystal to engineer its out-of-plane reflectivity, desired for efficient signal transduction of mechanical motion to light.
RESUMEN
Optical microresonators attract strong interest because of exciting effects and applications ranging from sensing of single atoms and molecules to quantum and nonlinear optics. For all this, control and tuning of the discrete resonances are vital. In resonators made of anisotropic materials that are beneficial for nonlinear-optical applications, anticrossings of ordinarily (o) and extraordinarily (e) polarized modes occur regularly. This effect is badly understood and harmful for mode control and tuning. We show that the anticrossings are inherent in the o- and e-modes because of the vectorial properties of Maxwell's equations. Within a novel pertubative approach employing a strong localization of the modes near the resonator rim, we have quantified the anticrossings. The values of avoidance gaps strongly exceeding the linewidths and selection rules for the interacting modes are predicted. The inferred values of the avoidance gaps are confirmed experimentally in resonators made of lithium niobate. Furthermore, based on theory, we have eliminated the anticrossings completely by spatially-controlled introduction of defects. This paves the way for unperturbed tuning of anisotropic microresonators.